1. Field of the Invention
The present invention relates to a pulse generator (pulse width modulation pulse generating circuit) for generating a pulse whose pulse width is controlled, and more specifically, to a data generator for generating input data for the pulse generator from source data corresponding to a predetermined desired pulse width in order to make the pulse generator, where a width of an output pulse in response to the input data is varied (or is not clear), generate a pulse having the desired pulse width.
2. Description of the Prior Art
A laser output modulating system has been suggested to control printing density in some laser printing apparatuses employed for digital copying machines, laser printers, etc. In this case, since the laser output is normally controlled by a pulse width, pulse width modulation (PWM) for converting printing data into a pulse having a pulse width corresponding to the laser output is required.
In high-speed laser printing apparatuses, the speed for processing printing data is very high, and the width of the pulses for transmitting data is extremely small. Further, when information on a printing density (gradation information) is added to the data representing each printing dot to control the pulse width according to the printing density, it is required to employ an extremely high-speed clock pulse as the reference clock pulse. For example, in order to express a printing density (gradation) with eight steps, a reference pulse, having a pulse width a eighth the pulse width when there is no gradation, has to be provided. However, when the high-speed reference clock pulse is used, peripheral devices of high-speed type are required, which largely increases the cost of the apparatuses as a whole. Therefore, it is undesirable to adopt such a high-speed reference clock pulse for popular office automation apparatuses.
Thereupon, a method has been considered where in order to perform the above-described pulse width modulation (PWM), a PWM pulse is generated not by using the high-speed reference clock pulse but by using delay devices. In the method, a reference pulse (normally, a clock pulse) is delayed by some delay devices, and a pulse having a small width (the delay time corresponds to the width) is obtained based on the difference between the delayed pulse and the reference pulse (not delayed).
In order to obtain a pulse having a given width from a PWM pulse generating circuit (hereinafter referred to as PWM circuit) employing delay devices, data on the number of steps of the delay devices, that is, how many delay devices of the PWM circuit should be activated are inputted to the PWM circuit.
However, since the characteristics of the delay devices are slightly different every PWM circuit (generally an integrated circuit (IC)), the width of the output pulses are different every PWM circuit (IC) even if the same step-number data are inputted. Moreover, even in a single PWM circuit, the width of the output pulses in response to the same input data (step-number data) varies according to a variation in temperature in the environment where the circuit is used and a variation in supply voltage.
Conventionally, the difference among each of the PWM circuits (IC) is adjusted by deciding a relation between the input and output every IC, which requires a lot of time and hands. Further, although the above-described method can remove the difference among each of the PWM circuits, the method could not cope with the variation in width of the output pulses according to a variation in environmental temperature and supply voltage.
On the other hand, when the PWM circuit is employed for a specific apparatus such as a laser printer, etc., the data (source data) for specifying the width of the pulses outputted by the PWM circuit cannot be varied according to the environmental temperature and supply voltage. For example, in a laser printer where printing density is controlled by laser output duration, data on printing density (hereinafter referred to as printing data) is inputted to the PWM circuit, and pulses having a predetermined width to be given to the laser generator are outputted. However, the printing data cannot be varied according to the difference among the PWM circuits or the variation in characteristic of the PWM circuits. Thereupon, a generator is required for generating input data for a pulse generating circuit based on source data (printing data in the above example) for making the PWM circuit generate pulses having a desired width.
FIG. 3 shows the the relation among the above-described printing data (source data), step-number data inputted to the PWM circuit, and width (a relative value when the reference pulse is 100%) of the pulses outputted by the PWM circuit. The three straight lines shown in the left half of the graph of FIG. 3 represent the relation between the inputted step-number data and the width of the outputted pulses with respect to three types of PWM circuits, that is, a PWM circuit CMIN consisting of delay devices having an extremely short delay time, a PWM circuit CMAX consisting of delay devices having an extremely long delay time and a PWM circuit CMD consisting of delay devices having an approximately middle delay time between the above-described two. For example, in order to obtain a pulse having a width of 50% from the PWM circuit CMIN consisting of delay devices having an extremely short delay time, data on a higher number of steps than that of the PWM circuit CMD having a middle delay time have to be inputted (that is, more delay devices have to be used). Further, since the delay times of the delay devices vary according to a change in environmental temperature and supply voltage even in a single PWM circuit as described above, the relation between the input data (step-number data) and the width of the output pulses changes from CMIN to CMAX according to circumstances.
As described above, even if the step-number data, that is, the printing data converted by a fixed relation L, are inputted into the PWM circuit where the width of the output pulses in response to input data varies, the width of pulses obtained thereby differs every PWM circuit or varies according to the environmental temperature and supply voltage. For example, even if printing data P50 corresponding to a density of 50% are inputted, the width of pulses (that is, the actual printing density) obtained thereby may be t1 which is lower than 50% (in CMIN), or may be 100% (that is, black) (in CMAX). Moreover, even if printing data P100 corresponding to a density of 100% (black) are inputted, there can be some cases where the width of pulses is t2 (that is, halftone) (in CMIN).
Another problem of the PWM pulse generating circuit employing delay devices is that a pulse having a width equal to or smaller than a predetermined minimum width or a width equal to or larger than a maximum width cannot be obtained from the pulse generating circuit. Since the principle of the pulse generation by the PWM circuit employing delay devices is as described above, naturally, the output pulse of the PWM circuit should be able to arbitrarily (if the number of steps of the delay devices is sufficient) take a value between 0 and 100% of a period T of the reference pulse (see (a) of FIG. 12). Actually, however, the output pulse is limited by the delay characteristic of an output buffer of the PWM circuit, so that, as shown in (b) of FIG. 12, a pulse having a width equal to or smaller than the predetermined minimum width tMIN is not outputted, nor is a pulse having a width equal to or larger than the maximum width tMAX. That is, even if the step-number data corresponding to a width equal to or smaller than tMIN are inputted into the PWM circuit, the output remains 0 (see (c) of FIG. 12); even if the step-number data corresponding to a width equal to or larger than tMAX are inputted, the output remains 1 (see (d) FIG. 12). FIG. 11 shows the relation between the step-number data inputted to the PWM circuit and the width of the output pulse (when the period T of the reference pulse is 100%). Therefore, in adjusting the PWM output, the above problem must be considered.